Blueberries in Pots

Growing blueberry bushes in containers

Q. I'm considering growing some blueberry bushes but don't know how they will do around here since it seems they grow up north. I have been considering growing them in outdoor containers if possible. What are my chances of success and are there any other tips you would offer?

A.Blueberries (Vaccinium spp.) grow very well in our area as long as you pay careful attention to soil preparation for them. They require an acidic, evenly moist yet well-drained soil rich in organic matter. If you choose to plant them in the ground, start with a soil test so that you can incorporate enough sulfur to get the pH into the preferred range of 4.5 to 5.5. Many gardeners spend a season getting the soil prepared and plant the following spring, but much depends on the existing soil conditions in your yard.

Winter cold can be the limiting factor when growing perennial or woody plants in containers in our climate. Many plants simply do not tolerate having their root systems completely frozen the way they can when grown above ground in a container, even if they are winter hardy here when planted in the ground. One way to get around that challenge is to choose plants that are much hardier than our coldest Zone 5a.

Blueberry Hardiness

Blueberries (Vaccinium spp.) are hardy to USDA Zone 3, so that is in your favor. Use the largest containers possible if you intend to leave them outdoors over the winter. Since blueberries are large plants – those that grow four to five feet tall and wide are considered compact growers - you will need to use large containers to support their growth anyway. At minimum something as large as a half whiskey barrel. It is critical that the containers have good drainage to avoid ice building up in them during freezing winter weather. You will have drill holes in the bottom of whisky barrels, if you choose to use them.

Another way to protect container-grown plants over the winter is to move them into an unheated garage in late fall. That allows you to control the water. You will need to water them periodically over the winter, perhaps only two or three times. It is important to monitor soil moisture through the winter storage period. Blueberries are shallow-rooted plants that require good drainage but resent drying out completely, too. They lack root hairs, the structure on most plant roots responsible for absorbing water and nutrients. This makes them extremely sensitive to too much or too little water.

Best Soil Mix for Blueberry Bushes

Your container soil mixture will be important to success. Be sure to fill the entire container with the soil mixture, rather than filling the bottom of the container with gravel or any other material. To make your soil mixture, start with a good soilless mix that contains a high percentage of peat moss. Amend it with an equal amount (by volume) of homemade compost (avoid mushroom compost here because it tends to have a high pH, around 8.0). You should also mix in small pine bark chips for some longer-lasting organic matter, about half the volume of potting mix or compost. Finish with medium chicken grit for sharp drainage, about one-quarter of the volume of potting mix or compost. Chicken grit is pH neutral and does not break down. Chicken grit is available at farm supply stores such as Agway. You can also mix in a coated release fertilizer such as Osmocote® Azalea, Camellia, Rhododendron Smart Release® Plant Food, according to label directions. This also contains an acidifying agent, which will help keep the pH in the desired range for blueberries.

Annual Care of Blueberries

Once the blueberries are planted, topdress the containers with the same fertilizer annually. Avoid the temptation to use more than the label rate of fertilizer; that could burn their shallow roots. You should also avoid less expensive, quick release fertilizers in containers because the relatively small soil volume cannot buffer the strong salts they contain. You may want to purchase a quick soil pH test kit to monitor the pH so you can amend it as needed to keep the pH low enough for the blueberries.

Sulfur is the preferred material to lower soil pH, but it moves very slowly through the soil profile. Sulfur is best incorporated prior to planting, if needed. Although I would typically recommend a Penn State soil test kit over the do-it-yourself types, translating the recommendations from “X amount of fertilizer per hundred feet of row” to an amount appropriate for a container is difficult.

Cross-Pollination of Blueberry Bushes

The typical recommendation is to plant two or three different blueberry cultivars to ensure good cross-pollination, which will increase fruit set. This also allows you to spread out the harvest, by choosing early, midseason, and late ripening cultivars. Some cultivars that would be good for your use include Patriot (early), Blueray (midseason), and Elliot (late).

Follow-up Care for Blueberries

Once they are planted, water them thoroughly and mulch with an inch or so of the same small pine bark chips you used to incorporate into your soil mix. This will help conserve soil moisture and moderate soil temperatures. It can also help keep weeds down, but weeds generally are not a big problem in containers. Place the containers in full sun, reasonably close to one another to ensure good pollination.

Once the plants begin to set fruit, you will have to cover them with netting to keep birds from devouring the ripening fruits. Monitor the blueberries’ water use and provide supplemental water as necessary. As the shrubs mature and their leaves spread over the edge of the containers, you may be surprised that you have to water, even if it rains, because the foliage will shed rainwater outside of the containers.